Viral agents known to cause hemorrhagic fever disease with high morbidity and mortality in humans, such as the Bunyaviruses - Crimean-Congo hemorrhagic fever virus, Rift Valley fever virus, and Hantaan virus or the Filoviruses -Ebola and Marburg virus, are priority pathogens of biodefense concern. In addition, there have been several outbreaks of newly discovered and/or emerging viruses which have also resulted in significant numbers of human fatalities, and these include the Flavivirus West Nile virus and the Henipaviruses Nipah and Hendra virus. These emerging viruses have also been classified as agents of biodefense concern because they possess characteristics making them suitable for weaponization. For all of these viruses there are neither approved vaccines nor therapeutics for the prevention of infection or disease. As a potential gap-filling approach for the rapid development of safe and efficacious antiviral therapies for these agents, we propose to identify, isolate and characterize neutralizing human monoclonal antibodies (nhMAbs) reactive to the native forms of the envelope glycoproteins (Envs) of these viruses. The development of these nhMAbs would provide a valuable battery of post-exposure or post-infection therapeutics to combat disease caused by these agents. The antibodies will be obtained by screening human phage display libraries against the viral Envs. Our Research Project will be comprised of four Sub-Projects. The unifying strategy of these projects will be the development of recombinant-based assays for measuring virus entry along with the production of native viral Envs as antigens for phage panning procedures. Sub-Project 1 will develop the assays and Env antigens for the Henipaviruses and Bunyaviruses; Sub-Project 2 will focus on the Flaviviruses; Sub-Project 3 will employ the Ebola and Marburg virus systems; while Sub-Project 4 will develop a second-generation alphavirus-based replicon system as a vaccine platform utilizing one or more of the viral Envs derived from the other Sub-Projects. The overall objectives of the Research Project will be to develop a battery of potent nhMAbs capable of being used as passive immunotherapy against several important viral pathogens, and develop a vaccine platform capable of eliciting a humoral response which includes the antibody reactivities observed by those nhMAbs developed by the other Sub-Projects. The overall Research Project's Specific Aims will be 1) Develop high-throughput recombinant assays for virus entry and develop recombinant viral Envs; 2) identify and isolate Fabs and scFvs capable of blocking virus entry by using recombinant viral Envs and/or functional pseudotyped or virus-like particles as antigens for phage panning; and 3) develop and test a novel alphavirus-based vaccine platform. In summary, the short-term goals of this research Program will be the development of nhMAbs that could be used for passive immunotherapy, while the long-term objective is the creation of a vaccine platform capable of eliciting similar antibody responses in an animal host. Both of these research goals are priorities as immediate and long-term objectives for biodefense research on viral hemorrhagic fevers and emerging viruses.